Cell hybrids will be prepared of a variety of mouse tumors and allogeneic macrophages. Growth rates of each tumor will be established in syngeneic mice. The effect of hybrid cells on the growth of the tumor will be determiend by injecting from 10 to the 3rd power to 10 to the 6th power living hybrid cells i.v. or i.p. at weekly intervals. Any immunogenic effect of an equal mixture of the hybrid cells will be compared with the effect of an equal mixture of the two parental cells not fused.